This invention relates to a catalytic alkylation process. It particularly relates to an improved process for the separation of the hydrocarbon and acid components present in the effluent from a catalytic alkylation zone. It specifically relates to an improvement to eliminate the overhead vapor apparatus of the HF regenerator, including the overhead condenser, overhead receiver, and overhead pump.
It is well known in the prior art that catalytic alkylation using hydrofluoric acid or sulfuric acid as the catalyst has become an important chemical tool for preparing alkylated hydrocarbons and derivatives thereof. The commercial and industrial demand for these products is exemplified with the demand for isoparaffin hydrocarbons and alkyl-substituted benzenes of gasoline boiling range and with the demand for alkyl-substituted aromatics suitable for conversion to surfactants, e.g., detergents, wetting agents, etc. The prior art process of alkylation generally is effected by contacting an isoparaffin hydrocarbon feed stock with an olefin hydrocarbon in the presence of a catalyst such as hydrofluoric acid in a suitable reaction vessel for conducting chemical reactions.
In practice, there have been numerous process schemes advanced by the prior art for accomplishing the alkylation reaction, but it is extremely difficult to achieve a process scheme which embodies all of the desirable features of a completely optimum reaction. Optimizing the alkylation reaction is complicated by the fact that the alkylation reaction if not carried out properly has many side reactions, such as polymerization, which destroys the effectiveness of the reaction and inhibits the production of commercial quantities of desired alkylate. Additionally, the reaction, in order to be carried out commercially, requires a tremendous amount of auxiliary equipment for the recovery of the alkylate product, for the regeneration and reuse of the excess catalyst, and for the recovery and reuse of the excess reactants which have passed through the reaction system.
The catalytic alkylation process to which the present invention is applicable consists of a process in which a mixture of hydrocarbons containing isoparaffins such as isobutane, isopentane, and the like, and olefins such as propylene, butenes, isobutenes, and the like, are mixed intimately in the presence of a strong acid catalyst, such as hydrofluoric acid or sulfuric acid at generally room temperature or lower for sufficient time to complete the reaction. The effluent from the reaction zone contains saturated isoparaffin hydrocarbons of higher molecular weight or boiling point than the isoparaffin in the original mixture. For convenience, these higher molecular weight isoparaffin hydrocarbons which comprise the reaction product from the alkylation zone are called "alkylate." Isobutane has been used almost exclusively because of its reactivity and availability to produce high quality alkylate product. In similar manner, among the olefins, butenes and propylenes have been used satisfactorily. In some cases it is desirable to use solely propylene or butene as the olefin reactant.
As is typical in most commercial chemical plants, the reaction between the isoparaffin hydrocarbon and the olefin hydrocarbon is performed with an excess of isoparaffin in the reaction zone. Accordingly, there is a large excess of the isoparaffin hydrocarbon remaining in the effluent from the reaction zone. Additionally, there is a significant quantity of C.sub.3 hydrocarbons which pass through the system, and for economy sake, must be recovered in as high yield as possible. In similar manner, it is desirable to recover for reuse the isoparaffin reactant in as high yield as possible, which is accomplished in an isoparaffin stripper, or, more specifically when isobutane is the isoparaffin, an isostripper.
In processes of the type referred to above there is also a need for periodic regeneration of the catalyst system. This is usually accomplished by prior art schemes by taking a stream of at least a portion of the acid catalyst, e.g., hydrofluoric acid, and passing it to a regeneration column wherein the regenerated catalyst is stripped with a light hydrocarbon, for example, hot or superheated vaporous isobutane. The purpose of this regeneration is to remove from the catalyst impurities such as water and acid soluble oils which accumulate in the system. These oils are of a polymeric composition which is in equilibrium with the alkylate hydrocarbon and heavy tar produced in the alkylation reaction. As used in this specification, these impurities and/or contaminants in the catalyst phase are for convenience lumped together and characterized as being material boiling above the boiling point of hydrogen fluoride acid.
The prior art processes for regenerating liquid catalyst such as hydrofluoric acid catalyst usually involve distillation schemes which present problems both from a process standpoint and from an apparatus standpoint. For example, since it is an acid system, the presence of water will cause severe corrosion problems in the regeneration column and in any condensing means associated therewith. Expensive, high quality alloy metallurgy is provided in the various apparatus associated with the regenerator to reduce the rate of corrosion found in this system, and even so, frequent replacement of equipment is not unusual. In addition, sufficient heat must be applied to the catalyst stream in order to vaporize the catalyst for recovery as a purified product. However, in the vaporization of this catalyst stream there will remain a non-vaporized residue of heavy organic diluent which tends to foul the tubes of the heat inducing means. Another problem present in the prior art process is the difficulty of providing sufficient stripping media so that the acid losses to the tar residue are minimized. If sufficient stripping media is passed into the regeneration column so that no acid will remain in the bottom product, there is frequently entrained overhead an excessive portion of heavy organic diluent which then contaminates the vaporized catalyst stream thereby creating additional fouling problems in the lines and condensing means associated with the regeneration system.
In the prior art, several means have been used to eliminate the HF regenerator overhead system, which can be described as the overhead condenser, overhead receiver, and overhead pump, or to combine that system with the overhead system of another fractionation apparatus. Thus it is seen in U.S. Pat. No. 3,349,146 that the regenerator overhead system is combined with the overhead system of a fractionator which strips HF from propane. Also in the prior art, in an isobutane stripper system wherein isobutane recycle is withdrawn from the isobutane stripper system as condensed overhead vapor saturated with HF, the overhead vapors of the HF regenerator are introduced into the overhead vapor conduit of the isobutane stripper upstream of the overhead condenser, thereby eliminating the regenerator overhead system. However, in the modern isobutane stripper, recycle isobutane is withdrawn as a side-cut from the isobutane stripper, and all overhead hydrocarbon product is withdrawn as feed to subsequent fractionation, i.e., depropanization. When the modern isobutane stripper came into use, it was considered desirable to separate the overhead systems of the regenerator and isobutane stripper in an effort to reduce incremental capital and operating costs of the depropanization fractionation, which were deemed greater than the incremental capital and operating costs of the separate regenerator overhead system.